A conventional alarm unit (photoelectric smoke sensor), which is known as a home fire alarm unit, detects a fire when the smoke density in a room exceeds a predetermined value and causes an alarm display lamp to blink, and it is provided with an alarm function for notifying, with an audio alert, the occurrence of the fire to people in the surrounding area.
Such a home alarm unit operates on a lithium battery serving as a power supply, and once a battery has been set therein, it is guaranteed that, for example, seven years of fire monitoring is possible without battery replacement.
In such an alarm unit, the battery voltage is, for example, 3 volts, which is too low for a voltage for light emission driving an LED, and therefore the battery voltage is boosted with a voltage booster circuit to, for example, 6 volts, which is a twofold voltage of the input voltage, to thereby cause the LED to emit light. Thus, even with a light emission in a short period of time in the order of microseconds, a sufficient amount of light is emitted from the LED, and light diffused by smoke particles flowing into a smoke-detection room is obtained.
FIG. 5 is a block diagram showing a light emission driving circuit of the conventional alarm unit, along with an MPU. In the light emission driving circuit 100 shown in the diagram, a constant voltage circuit 104 is provided so as to be series with a battery 102, to thereby charge, at a constant voltage, a large-capacity capacitor 106. To the capacitor 106, there are connected, via a switching device 108 such as transistor and FET, a large capacity capacitor 110 and a resistor 111, and there is further connected a resistor 112 so as to be parallel with a serial circuit formed with the switching device 108 and the capacitor 110. With such a connection structure, a charge pump circuit is configured. To the secondary side of the capacitor 110, an LED 116 which is a light emitting device is connected via a switching device 114.
The battery 102, in a state where the switching devices 108 and 114 are OFF, charges, via the resistor 112, the capacitor 110 at a constant voltage. When, for example, a fire hazard detection cycle of every 10 seconds is reached, the MPU 118 turns the switching device 108 ON and serially connects the capacitor 110 to the capacitor 106 to thereby boost the voltage to a twofold voltage of the input voltage. At the same time, the MPU 118 turns the switching device 114 ON, and thereby applies the voltage boosted in the serial connection of the capacitors 106 and 110, to the LED 116, causing it to emit light.
Light from the LED 116 collides with smoke particles flowing into the smoke-detection room, and is diffused. This diffused light is received on a photodiode 120, which is a light receiving device, to be converted into an imperceptible light reception signal, and is amplified in a received light amplifying circuit 122 in synchronization with the light emission drive. Then, it is input to the MPU 118 and is further converted, through AD conversion, into received light data. If this received light data exceed a predetermined fire hazard level, then an alarm output circuit 124 is operated to output a fire hazard alarm.
FIG. 6 is a block diagram showing a light emission driving circuit of another alarm unit, along with an MPU. The light emission driving circuit 100 does not require a constant voltage circuit; and with a battery 102, a large capacity capacitor 126 is charged at a constant voltage. To the capacitor 126, there is serially connected, via a switching device 128, a small capacity capacitor 130, and there is further connected a backflow preventing diode 132 so as to be parallel with the serial circuit formed with the switching device 128 and the capacitor 130. Moreover, a switching device 134 is serially connected to the capacitor 130.
To the secondary side of the capacitor 130, there is connected, via a backflow preventing diode 136, a large capacity capacitor 138, and there is further connected, via a switching device 140 and a constant voltage circuit 142, an LED 116, which is a light emitting device.
The battery 102, in a state where the switching device 128 is OFF and the switching device 134 is ON, charges the small capacity capacitor 130 via the backflow preventing diode 132. After this, the MPU 118 repeats operations of turning ON the switching device 128, switching OFF the switching device 134, serially connecting the capacitor 130 to the capacitor 126, and charging, at a boosted voltage, the large capacity capacitor 138 via the backflow preventing diode 136. When a fire hazard cycle of every 10 seconds is reached, the switching device 140 is turned ON, and the boosted voltage is applied to the LED 116, causing it to emit light.
As described above, by repeatedly performing charging with the small capacity capacitor 130, the amount of a single charge transfer is suppressed, thereby reducing the capacity of the switching devices 128 and 134. Moreover, the constant voltage circuit 142 is only operated when light emission is performed.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2007-011828    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2006-350412    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2007-179367    [Patent Document 4] Japanese Unexamined Utility Model Application, First Publication No. H05-008696